The least common multiple of two numbers is 3780, and the greatest common divisor is 18. Given that one of the numbers is 180, what is the other number?
Answer: We use the identity $\gcd(a,b) \cdot \mathop{\text{lcm}}[a,b] = ab$ for all positive integers $a$ and $b$.  We are told that $\gcd(a,b) = 18$, and $\mathop{\text{lcm}}[a,b] = 3780$, so $ab = 18 \cdot 3780$.  If one number is 180, then the other number is $18 \cdot 3780/180 = \boxed{378}$.